With the current popularity of portable communication devices and developed semiconductor technologies, high level integration is becoming a trend in semiconductor fabrications. By integrating different transistors into a single semiconductor device within one fabrication process, the final package size and cost will be reduced. In different applications, these integrated transistors may implement different functions. Isolation of these integrated transistors is desired for different functions.
Pseudomorphic high electron mobility transistors (pHEMTs), due to their high gain up to millimeter-wavelengths and their enhanced noise and power performance, are widely used in high speed communication devices. To implement different functions with optimal performance, the pHEMTs may operate in different modes. For instance, the pHEMTs may operate in an enhancement mode to optimize low noise amplifier (LNA) performance, or the pHEMTs may operate in a depletion mode for high-speed switch implementations. However, a conventional pHEMT structure has a single channel that both the enhancement mode and the depletion mode share. Consequently, it is rarely possible to optimize the performance of both the enhancement mode and the depletion mode independently.
Accordingly, there remains a need for improved semiconductor device designs to utilize the advantages of pHEMTs while optimizing the performance of both the enhancement mode and the depletion mode independently. In addition, there is also a need to keep the size and cost of the final products effective.